1. Field of the Invention
The present invention relates to hot-melt adhesive compositions and to processes for preparing the same.
2. Description of the Prior Art
Recently, hot-melt adhesives have been developed as non-solvent type adhesives which permit high speed processing and which have excellent adhesive properties suitable for mass production. Hot-melt adhesives are mainly used for processing paper goods which can be collected after use and reclaimed. However prior hot-melt adhesives as are commonly used have serious defects from the viewpoint of the economics of collecting and reclaiming the used paper because they do not dissolve or disperse in water, and, therefore, are difficult to reclaim or they deteriorate to lower the commercial value of the reclaimed paper.
As water soluble polymers which melt by heating, vinyl acetate-vinyl pyrrolidone copolymers and water soluble nylons are known. However, they cannot be used for bookbinding or sealing corrugated paper and cartons because they have so high a viscosity that commonly used applicators cannot be used to apply the same. Further, they have insufficient water dispersibility or low adhesive properties. Further, remoistenable hot-melt adhesive compositions containing partially hydrolyzed polyvinyl acetates having a degree of hydrolysis of about 50 to about 85 mol% as a main ingredient, which can be used for envelopes, postage stamps, gum tapes or wall paper, etc., have been described in U.S. Pat. No. 3,597,264. In this patent, polyvinyl alcohol having an acetate group content of about 15 to about 50 mol% (the adhesive ingredient) is produced by acid hydrolysis of an aqueous dispersion of polyvinyl acetate (hereinafter referred to as PVAc). In general, acid hydrolysis has the following defects, as described in "Polyvinyl Alcohol", pages 91 - 96, edited by C. A. Finch, (published by a Wiley-Interscience Publications (1973)): A large amount of acid catalyst and a long period of time are required to obtain a desired degree of hydrolysis because the rate of the hydrolysis reaction is very low as compared with that of alkaline hydrolysis. Further, it causes a deterioration of the quality of the polyvinyl alcohol (hereinafter referred to as PVA) because a large amount of salts are formed by neutralization with alkalis after the reaction because of the use of a large amount of acid catalyst, which salts are retained in PVA. Further, it is necessary to remove a large amount of water by evaporation in order to obtain PVA because the hydrolyzed product formed by the hydrolysis in an aqueous medium dissolves in water. As described above, acid hydrolysis is economically disadvantageous as compared with alkaline hydrolysis. Therefore, acid hydrolysis has not be utilized hitherto for the industrial production of PVA.
Further, the arrangement of residual acetate groups in the PVA molecules obtained by acid hydrolysis is random, as compared with that of PVA obtained by alkaline hydrolysis. Accordingly, the PVA obtained by the former process has a low melting point, a low melt viscosity and high hygroscopicity, and, while suitable for remoistenable adhesives, its adhesive strength is insufficient for use for bookbinding, sealing corrugated paper or cartons, etc.
Particularly, PVA obtained by acid hydrolysis has extremely low adhesive strength at a high temperature and high humidities.
On the other hand, PVA obtained by known alkaline hydrolysis processes is difficult to melt, has a high melting point, a high melt viscosity and poor heat stability and undergoes foaming upon melting. In order to lower the melting point, it is necessary to add a large amount of a plasticizer or plasticizers. However, adhesive strength is deteriorated by the addition of a plasticizer(s), and thus the resultant PVA has comparatively low adhesive strength and cannot even be used as a remoistenable adhesive which does not require high adhesive strength.